1. Field of the Invention
The present invention relates to an optical sensing circuit for an optical detecting portion or the like of an integrated circuit for automatic focusing of a camera, and more particularly to a technique for converting a photoelectric current in a photodiode into a varying voltage and for detecting the intensity of light by using this voltage variation.
2. Description of the Prior Art
A conventional optical sensing circuit is shown as an example in FIG. 1. The circuit comprises a photodiode 1 as a photoelectric current detecting means, a reset transistor 3 as a voltage setting means, and a comparator 4 as a voltage comparing means. One electrode of the photodiode 1 (cathode) is connected to a power source voltage V.sub.DD, and the other electrode thereof (anode) is connected to an initial voltage V.sub.0 via the reset transistor 3 and is connected to the comparator 4.
In this optical sensing circuit, when the reset transistor 3 is turned on, the anode of the photodiode 1 is set to the initial voltage V.sub.0, and a reverse biased voltage (V.sub.DD -V.sub.0) is applied to the photodiode 1. As shown in FIG. 2, when an input signal S.sub.G1 is switched from level H to level L and the reset transistor 3 is cut off, a photoelectric current I flows from the power voltage source to the anode of the photodiode 1 according to the intensity of light irradiated onto the photodiode 1. This causes an equivalent capacity 2 to be discharged, the equivalent capacity being composed of a junction capacity of the photodiode 1, a drain capacity of the reset transistor 3, an input capacity of the comparator 4, and the like. During this discharge, the input voltage of the comparator 4 increases according to the following expression (1): EQU V=V.sub.0 +I.multidot.t/C (1)
where, C is the capacitance of the equivalent capacity 2, t is the time passing from the time when the reset transistor 3 is cut off, and the equivalent capacity 2 functions as an integral capacity of a current-to-voltage converting means for converting of photoelectric current into a voltage.
If the input voltage rises according to expression (1) described above and exceeds a reference voltage V.sub.ref, the output voltage of the comparator 4 is inverted as shown in FIG. 2. The time t.sub.s, between the switching time of the input signal and the time when the output voltage of the comparator 4 is inverted (referred to below as the reverse time period t.sub.s), has the following relationship (2) with the photoelectric current I: EQU t.sub.s =(V.sub.ref -V.sub.O).multidot.C/I. . . (2)
Consequently, the magnitude of the photoelectric current, or the intensity of light irradiated onto the photodiode 1 can be detected by measuring the reverse time period t.sub.s.
In order to improve in the sensitivity of the above conventional optical sensing circuit, there are three methods, (1) decreasing the value of (V.sub.ref -V.sub.O), (2) decreasing the capacitance C of the equivalent capacity 2, and (3) increasing the photoelectric current I.
However, If the value of (V.sub.ref -V.sub.O) is decreased to a small value, unevenness of the light sensitivity is increased due to an offset voltage of the comparator 4. This makes it difficult to obtain uniform sensitivity characteristics. With regard to the method (2) which tries to reduce the capacitance C of the equivalent capacity 2, the capacitance C is affected by the total capacity of the junction capacity of the photodiode 1, the drain capacity of the reset transistor 3 and the input capacity of the comparator 4. This will cause not only unevenness of the capacitance but also a reduction of the SN ratio of the photodiode 1. These problems are particularly conspicuous when the junction capacitance is decreased. Therefore, the minimum value of the capacitance C is restricted.
Moreover, in the method (3) which tries to increase the photoelectric current I, an improvement in a conversion efficiency of the photodiode 1 is restricted. This makes it difficult to obtain the conversion efficiency higher than the conventional one. In other words, it is very difficult to improve the light sensitivity of the conventional optical sensing circuit by using any one of the above methods (1) to (3).
In addition, the switching noise of the reset transistor 3 or the like will induce fluctuations of the initial voltage V.sub.O. As a result, the accuracy of the intensity of light obtained by using the reverse time period ts deteriorates.
Furthermore, even if the improvement in the sensitivity for the light intensity in a predetermined range is achieved by using the above methods, the dynamic range for the light intensity is restricted. The reason is that if the intensity of light is large and far beyond the range, the reverse time period ts shown in expression (2) becomes small and the resolution of the light intensity is decreased.